The CSS Painting API — part of the CSS Houdini umbrella of APIs — allows developers to write JavaScript functions that can draw directly into an element's background, border, or content.

Essentially, the CSS Painting API contains functionality allowing developers to create custom values for paint(), a CSS <image> function. You can then apply these values to properties like background-image to set complex custom backgrounds on an element.

For example:

aside {
  background-image: paint(myPaintedImage);
}

The API defines a worklet that can be used to programmatically generate an image that responds to computed style changes. To find out more about how this is used, consult Using the CSS Painting API.

PaintWorkletGlobalScope

The global execution context of the paint worklet.

PaintRenderingContext2D

Implements a subset of the CanvasRenderingContext2D API. It has an output bitmap that is the size of the object it is rendering to.

PaintSize

Returns the read-only values of the output bitmap's width and height.

The following example creates a list of items with a background image that rotates between three different colors and three widths. In a supporting browser you will see something like the image below.

To achieve this we'll define two custom CSS properties, --boxColor and --widthSubtractor.

In our worklet, we can reference these custom properties.

registerPaint(
  "boxbg",
  class {
    static get contextOptions() {
      return { alpha: true };
    }

    /*
     use this function to retrieve any custom properties (or regular properties, such as 'height')
     defined for the element, return them in the specified array
  */
    static get inputProperties() {
      return ["--boxColor", "--widthSubtractor"];
    }

    paint(ctx, size, props) {
      /*
       ctx -> drawing context
       size -> paintSize: width and height
       props -> properties: get() method
    */

      ctx.fillStyle = props.get("--boxColor");
      ctx.fillRect(
        0,
        size.height / 3,
        size.width * 0.4 - props.get("--widthSubtractor"),
        size.height * 0.6,
      );
    }
  },
);

We used the inputProperties() method in the registerPaint() class to get the values of two custom properties set on an element that has boxbg applied to it and then used those within our paint() function. The inputProperties() method can return all properties affecting the element, not just custom properties.

HTML

<ul>
  <li>item 1</li>
  <li>item 2</li>
  <li>item 3</li>
  <li>item 4</li>
  <li>item 5</li>
  <li>item 6</li>
  <li>item 7</li>
  <li>item 8</li>
  <li>item 9</li>
  <li>item 10</li>
  <li>item 11</li>
  <li>item 12</li>
  <li>item 13</li>
  <li>item 14</li>
  <li>item 15</li>
  <li>item 16</li>
  <li>item 17</li>
  <li>item</li>
</ul>

CSS

In our CSS, we define the --boxColor and --widthSubtractor custom properties.

li {
  background-image: paint(boxbg);
  --boxColor: hsl(55 90% 60% / 1);
}

li:nth-of-type(3n) {
  --boxColor: hsl(155 90% 60% / 1);
  --widthSubtractor: 20;
}

li:nth-of-type(3n + 1) {
  --boxColor: hsl(255 90% 60% / 1);
  --widthSubtractor: 40;
}

JavaScript

In our <script> we register the worklet:

CSS.paintWorklet.addModule("boxbg.js");

Result

While you can't play with the worklet's script, you can alter the custom property values in DevTools to change the colors and width of the background image.

• Using the CSS Painting API
• CSS Typed Object Model API
• CSS Houdini